1. Field of the Invention
This invention relates to feedback control systems, particularly for machine control.
2. Prior Art
Numerical control systems are used to control machines such as machine tools. Digital commands are input with a punched tape to describe the part to be cut on a machine such as a lathe machine. These commands are accessed by the numerical control system and are executed to drive the machine to automatically cut the part.
Prior art numerical control systems have been designed around special purpose digital devices where operations such as logic, control, and computations are performed with special purpose logic. Because of the limited capability of these special purpose numerical control systems, interaction between the contour generator and servo is usually precluded, permitting only open loop incremental commands from the contour generator to the servo. One exception is for threading operations, where an incremental pickoff is used to sense incremental changes in spindle position. These incremental feedback pulses are used by well known incremental digital differential analyzers to synchronize the contour with the spindle. Communication is in incremental form; thereby precluding the detection of accumulating errors such as with noise causing loss or introduction of incremental pulses; precluding the use of stored program data processors because of the high iteration rates associated with incremental computations; reducing reliability due to the relatively unreliable operation of incremental pickoffs; reducing threading speed due to the relatively low rotational velocity of incremental pickoffs; increasing costs due to the relatively high cost of incremental pickoffs as compared to resolver type pickoffs and other such disadvantages.
Contouring controls command the rectilinear translational axis to move relative to the independent variable, which is time. A distance command to an axis for each period of time defines an axis velocity, which is that distance divided by that period. Distance commands to a plurality of machine axes for each period of time defines not only the velocity of each axis, but the relative slope of the multi-dimensional contour which is defined by the relative magnitudes of those distance or velocity commands.
Precise turning operations such as thread cutting on a lathe requires a multi-dimensional contour that includes the spindle rotational degree of freedom. The translational axes have high performance, high accuracy drives that permits close tracking of the commands. Unfortunately, the spindle drive is typically implemented with a high power but low accuracy drive. Therefore, it is not possible to provide precise synchronized spindle motion by controlling the spindle motor. One solution to the problem has been to use the spindle motion as the independent variable for the contouring operation, with the high performance translational motions slaved to the spindle motion. An incremental sensor is usually geared to the spindle to generate an output pulse for each increment of spindle rotational displacement. These feedback increments are used as the independent variable inputs to the incremental digital differential analyzer contouring devices used in prior art systems.
Multi-speed resolver pickoffs have been used for absolute position capability in prior art systems, where the various resolvers are geared for different ranges of the axis position and are switched into the servo loop; but provide no feedback to the control portion of the system, precluding interactive control.
In the prior art, stored program data processors have been used to command physical systems, such as with machine tools, but these commands were not real time path defining commands. The data processor merely generated end point commands to non-computerized numerical control systems, where the non-computerized numerical control systems provided the interpolation function to generate the multitudes of intermediate incremental path defining commands to the machine tool subsystem. Prior art systems that have been used to generate end point commands to non-computerized numerical control systems are the General Electric Commandir system and the Sundstrand Omnicontrol system.